The cellular mechanisms that are involved in training-induced adaptations of the contractile machinery of the myocardium are not well understood. Work in this proposal tests hypotheses regarding contractile adaptations at the level of the single cardiac myocyte. The contractile properties of single cardiac myocytes isolated from the hearts of both exercise trained rats and sedentary controls will be measured. The effect of training on the calcium sensitivity of isometric force and kinetic properties of force (rate of force development and rate of relaxation) will be determined. The effect of exercise training on the maximal shortening velocity and the force-velocity relationship of cardiac myocytes will also be determined. Changes in contractile function of the myocytes will be correlated with the isoform composition of contractile proteins and their phosphorylation state.